Mounting a HF antenna

M0LMK

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I've seen several images of large HF whips mounted on cars. Some are at roof level, some are mounted on the tow bar and others are mounted 1/2 way between. Am I correct in thinking that a roof mount is the most efficient performance wise?
 

G0DZB

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I believe the main issue is getting a good earth connection to the car body at the base of the antenna. This is why a single mag-mount does not work on the lower HF bands as it doesn't provide enough capacitive coupling into the roof to provide a low impedance earth at the feed point.

Coincidentally I've been experimenting with providing additional earthing for a mag-mount on HF by adding earthing straps to a couple of conveniently located bolts that attach the boot lid to the body of my estate. Initial tests are encouraging. Running 10W on 10Mhz I measured 250mA in each earth strap, and it also greatly reduced the current in the wire that earths the ratio case to the car body.
 

M0LMK

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That sounds interesting Peter.

I'm going to be putting a full length roof rack on the landrover soon and was thinking about having a bracket welded on. The rack attaches to the car via 8 large bolts and I may put some aluminum tread plate on the floor of the roof rack.

How are you measuring the current flow across your earth straps?
 

WD5IKX

Member #46
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Go to the K0BG website for anything you need to know about HF mobile. K0BG.com

John
WD5IKX
 

mu0chn

Member #10
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What antennas are you planning on using and also what model land Rover.
If it is a 90/110 then the top of the front wing would be my choice.
On my Volvo 940 estate, I have a 3/8 mount that bolts through a vertical pannel. Drill 2 holes, one is slightly bigger and has an insulating washer and the coax connects to that.
And then there is this
 

G0DZB

Member #84
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Ha ha.... Folding up loop is great until someone forgets that they put a canoe on the roof rack :)
 

M0LMK

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What antennas are you planning on using and also what model land Rover.
If it is a 90/110 then the top of the front wing would be my choice.
On my Volvo 940 estate, I have a 3/8 mount that bolts through a vertical pannel. Drill 2 holes, one is slightly bigger and has an insulating washer and the coax connects to that.
And then there is this

It's a homebrew bug catcher style antenna (9' long with the short whip or 13' long with the long whip so only for static use!) and a Freelander. I'm picking up a full length roof rack in a few days so I can mount it on that if higher is better.
 

M0LMK

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I have now and I'm not sure if its it's for real or a joke. RF ground is not the same as electrical ground (which is what he tests in that video), the boot mount that he said is great would be useless with a HF whip with it's 4 tiny set screws holding it on and the roof rack mount was a roof bar mount. Was an interesting watch though so thanks :)

Here's a picture of the roof rack I'll be fitting. The only difference is that mine will have a rubber covered aluminium floor covering the entire rack floor. I think I'll also bond it to the car with a few 1" copper straps.

91-NF-FR.jpg
 

mu0chn

Member #10
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Here's a picture of the roof rack I'll be fitting. The only difference is that mine will have a rubber covered aluminium floor covering the entire rack floor. I think I'll also bond it to the car with a few 1" copper straps.

View attachment 695

I think that if the aluminium floor has good contacts where it is bolted to the roof rack you will have a good start. Years ago my Dad had 5 Dexion shelves bolted together and a Mobile whip mounted in the middle.

Have you looked at this site loads of good stuff.
 

WD5IKX

Member #46
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I just got my "Bug Eliminator" antenna mounted on my new truck. The tip is 13' 8" above the ground so mounting in the middle of the roof is not an option. (o: Where it is mounted on the back of the truck I can still get into my car park. I am now working on running the coax, mounting the shunt coil and improving the ground bonding to the body. When I had this antenna on my old vehicle it was great giving me contacts all over the globe. I am looking forward to being mobile again.

Bug Eliminator on Ram 1500-2-sm.JPG

John
WD5IKX
Texas
 

M0LMK

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That looks great John. Is it homebrew? Looks like it could be an MFJ coil in the middle.
 

WD5IKX

Member #46
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Hi Matt

Yes it is home brew. I made one earlier with the MFJ 67 coil and it works great on 30, 20, 17, 15, 12, and 10 meters. I called it the "Bug Whacker".
I bought the coil parts for the "Bug Eliminator" from another ham and the Mast is from Hustler and the 72" whip is from MFJ.
The new truck required a ball mount which I got from Beedlove. https://breedlovemounts.com/
The Bug Eliminator operates on 40, 30, 20, 17, and 15 meters with the 72" whip. If I want 12 and 10 meters I usually swap to the "Bug Whacker" coil.

Bug Eliminator-1-sm-label.jpg5 Band Bug Whacker-4.jpgBug Whacker

John WD5IKX
Texas
 

mu0chn

Member #10
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I've seen several images of large HF whips mounted on cars. Some are at roof level, some are mounted on the tow bar and others are mounted 1/2 way between. Am I correct in thinking that a roof mount is the most efficient performance wise?

How you getting on with your mobile mount Matt.
 

M0LMK

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It's moving along. I've been out and purchased some new SS bolts and washers to hold the rack onto the roof brackets a small section of steel plate. The plan is to make 4 brackets that should sandwhich between the roofrack and the mounting legs. I'll get some pictures up when the brackets are done.
 

WD5IKX

Member #46
New member
I am making some progress. I made a shunt coil and attached it today to improve loading on 40 meters. I did some tuning and it looks like it is doing the job. I need to do more bonding before I finish tuning. I am waiting on some wiring parts so I can finish running the DC from the battery. Hope it all comes together soon....I miss being mobile.

BE Shunt coil-1.JPG

John WD5IKX
 

WD5IKX

Member #46
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I hooked up my FT-450 with a battery in the truck to test the antenna.....Well it seems to be working since I got 59 reports from HG2DX in Hungary and HK1NA in Columbia on 20 meters and M0HDX Jim in England on 17 meters and this was running only 50 watts. Now I have to get all the power wiring done and finish bonding which I hope will cure the ignition noise I am getting. I am anxious to get this project done.

John WD5IKX
Texas, USA
 
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mu0chn

Member #10
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How's it going Matt, are you mobile yet.
My Volvo est broke down so I bought a Hijet van but there is not much room for anything in that.
Looking at a Nissan Cargo now, more room for work gear and 2nd battery
 

M0LMK

Member #2
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Allmost...

I've got the second battery in and the 857 in place. Just need to run the power to it and cable in split charge system in which should be some time next week. I've also got an ATAS 120 for HF (not very efficient but convenient for on the move) and a spare wheel bracket which should be delivered tomorrow.

Will post pictures as soon as it's done.
 
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